Types of Graphs:

1. Null Graph: A null graph is defined as a graph which consists only the isolated vertices.

Example: The graph shown in fig is a null graph, and the vertices are isolated vertices.

2. Undirected Graphs: An Undirected graph G consists of a set of vertices, V and a set of edge E. The edge set contains the unordered pair of vertices. If (u, v)∈E then we say u and v are connected by an edge where u and v are vertices in the set V.

Example: Let V = {1, 2, 3, 4} and E = {(1, 2), (1, 4), (3, 4), (2, 3)}.Draw the graph.

Solution: The graph can be drawn in several ways.

Two of which are as follows:

3. Multigraph: If in a graph multiple edges between the same set of vertices are allowed, it is known as Multigraph. In other words, it is a graph having at least one loop or multiple edges.

4. Directed Graphs: A directed graph or digraph G is defined as an unordered pair (V, E), where V is the set of points called vertices and E is the set of edges. Each edge in the graph G is assigned a direction and is identified with an ordered pair (u, v), where u is the initial vertex, and v is the end vertex.

Example: Consider the graph G = (V, E) as shown in fig. Determine the vertex set and edge set of graph G.

Solution: The vertex and edge set of graph G =(V, E) is as follow

5. Undirected Complete Graph: An undirected complete graph G=(V,E) of n vertices is a graph in which each vertex is connected to every other vertex i.e., and edge exist between every pair of distinct vertices. It is denoted by K_n.A complete graph with n vertices will have edges.

Example: Draw Undirected Complete Graphs k₄and k₆.

Solution: The undirected complete graph of k₄ is shown in fig1 and that of k₆is shown in fig2.

6. Connected and Disconnected Graph:

Connected Graph: A graph is called connected if there is a path from any vertex u to v or vice-versa.

Disconnected Graph: A graph is called disconnected if there is no path between any two of its vertices.

Example: Consider the graph shown in fig. Determine whether the graphs are

(a)Disconnected Graph

(b)Connected Graph.

Also, write their connected components.

Solution:

(i) The graph is shown in fig is a Disconnected Graph, and its connected components are

            {V_1,V_2,V_3,V₄},{V_5,V_6,V_7,V₈} and {V_9,V₁₀}.

(ii) The graph shown in fig is a Disconnected Graph and its connected components are

            {V_1,V₂},{V_3,V₄},{V_5,V₆},{V_7,V₈},{V_9,V₁₀}and {V_11,V₁₂}.

(iii) The graph shown in fig is a connected graph.

7. Connected Component: A subgraph of graph G is called the connected component of G, if it is not contained in any bigger subgraph of G, which is connected. It is defined by listing its vertices.

Example: Consider the graph shown in fig. Determine its connected components.

Solution: The connected components of this graph is {a, b, c}, {d, e, f}, {g, h ,i} and {j}.

8. Directed Complete Graph: A directed complete graph G = (V, E) on n vertices is a graph in which each vertex is connected to every other vertex by an arrow. It is denoted by K_n.

Example: Draw directed complete graphs K₃ and K₅.

Solution: Place the number of vertices at the appropriate place and then draw an arrow from each vertex to every other vertex as shown in fig:

9. Complementary Graph: The complement of a graph G is defined to be a graph which has the same number of vertices as in graph G and has two vertices connected if and only they are not related in the graph.

Example: Consider the graph G shown in fig. Find the complement of this graph.

Solution: The complement of the above graph is shown in Fig:

10. Labeled Graphs: A graph G=(V, E) is called a labeled graph if its edges are labeled with some name or data. So, we can write these labels in place of an ordered pair in its edges set.

Example: The graph shown in fig is labeled graphs.

            G= {{a, b, c, d}, {e_1,e_2,e_3,e₄}}

11.Weighted Graphs: A graph G=(V, E) is called a weighted graph if each edge of graph G is assigned a positive number w called the weight of the edge e.

Example: The graph shown in fig is a Weighted Graph.